In a background switching power-supply apparatus, a starting circuit, which supplies a starting current to a control circuit controlling a switching operation when a DC power source is turned on, includes a constant current circuit (for example, see JP-A-2003-333840).
As shown in FIG. 6, the background switching power-supply apparatus includes a capacitor C1, a starting circuit 1, a transformer T having a primary winding P1, a secondary winding S1 and a tertiary winding P2, a switching element Q10 configured by MOSFET, for example, a resistor R10 detecting a current flowing through the switching element Q10, a control circuit 3 for controlling on-and-off switching of the switching element Q10, a first rectifying-and-smoothing circuit including a capacitor C11 and a diode D11, a second rectifying-and-smoothing circuit including a capacitor C10 and a diode D10, and a detection circuit 7.
The capacitor C1 equivalently represents a capacitor that is provided to an input of a switching power supply, for example, a smoothing capacitor or the like in the case of rectifying-and-smoothing an AC power source to input to the switching power supply. Even when the DC power source E that is input to the switching power supply is cut off, voltage does not drop to zero immediately due to the capacitor C1. The starting circuit 1 is connected between a connection point of a positive terminal of the capacitor C1 with one end of the primary winding P1 of the transformer T and a power input terminal VCC of the control circuit 3, and the DC power source E is intermittently or constantly applied to the starting circuit 1. The control circuit 3 is started at a starting voltage Von (18V, for example) and is stopped at a stop voltage Voff (9V, for example). The control circuit 3 controls an output voltage as a predetermined voltage by turning on-and-off of the switching element Q10, based on an output voltage Vout detected by the detection circuit 7.
The starting circuit 1 is provided with a series circuit including a resistor R1, a constant current circuit CC1, a switch SW1 and a diode D1, and a comparator CP. The series circuit including the resistor R1, constant current circuit CC1, switch SW1 and a diode D1 is connected between the connection point of the positive terminal of the capacitor C1 with one end of the primary winding P1 of the transformer T and the power input terminal VCC of the control circuit 3. In the comparator CP, a non-inverting input terminal is connected to the connection point between the cathode of the diode D1 and the power input terminal VCC of the control circuit 3, and an inverting input terminal is connected to a reference power supply Vr1. The output terminal of the comparator CP is connected to a contact point of the switch SW1. The comparator CP has a hysteresis characteristic, and it outputs an H-level when the non-inverting input terminal becomes the starting voltage Von (18V, for example) of the control circuit 3 and outputs an L-level when the non-inverting input terminal becomes the stop voltage Voff (9V, for example) of the control circuit 3.
An operation of the switching power supply configured as described above will be described with reference to FIG. 7.
First, when the DC power source E is input at time point t1, in the starting circuit 1, the input voltage Vst is applied to the constant current circuit CC1 via the resistor R1. At this time, since the output of the comparator CP is at the L-level, the switch SW1 is in a state of having been turned on. Thus, a predetermined constant current Ic (2.5 mA for example) flows as the starting current Ist by the constant current circuit CC1, the capacitor C10 is charged through the diode D1, and thereby the voltage of the capacitor C10 rises. The voltage of the capacitor C10 is supplied to the power input terminal VCC of the control circuit 3 thereby becoming a power-supply voltage Vcc of the control circuit 3.
First, since the power-supply voltage Vcc is lower than the starting voltage Von of the control circuit 3, the output of the comparator CP is at the L level, and the switch SW1 is continued in an ON-state. When the power-supply voltage Vcc reaches the starting voltage Von at time point t2, the control circuit 3 is started. When the control circuit 3 is started, the drive signal Dry is transmitted from the control circuit 3, and then the switching element Q10 starts to turn on-and-off. Therefore, the DC power source E is intermittently applied to the primary winding P1 of the transformer T, and a voltage is induced in the secondary winding S1. The voltage generated in the secondary winding S1 is rectified and smoothed in the diode D11 and capacitor C11, and the output voltage Vout is supplied to the load 5. The output voltage Vout supplied to the load 5 is compared with a reference voltage in the detection circuit 7, and an error signal is input to the control circuit 3. The control circuit 3 generates the drive signal Dry of duty based on the error signal, thereby turning on-and-off the switching element Q10.
Further, at timing when the power-supply voltage Vcc becomes the starting voltage Von at time point t2, the output of the comparator CP becomes from the L-level to the H-level, and the switch SW1 is turned off. Thus, charging of the capacitor C10 by the starting circuit 1 is stopped. Then, the DC voltage, which is obtained by rectifying-and-smoothing the voltage generated in the tertiary winding P2 of the transformer T by the diode D10 and capacitor C10, is supplied to the control circuit 3.